/Engine/Source/Runtime/Engine/Public/MaterialShared.h
Gets the shader map usage of the material, which will be included in the DDC key. This mechanism allows derived material classes to create different DDC keys with the same base material. For example lightmass exports diffuse and emissive, each of which requires a material resource with the same base material.
virtual [EMaterialShaderMapUsage::Type](API\Runtime\Engine\EMaterialShaderMapUsage__Type) GetShaderMapUsage() const